DETROIT — Detroit on Thursday became the largest city in U.S. history to file for bankruptcy, as the state-appointed emergency manager filed for Chapter 9 protection.

Kevin Orr, a bankruptcy expert, was hired by the state in March to lead Detroit out of a fiscal free-fall and made the filing Thursday in federal bankruptcy court.

A number of factors — most notably steep population and tax base falls — have been blamed on Detroit's tumble toward insolvency.

Detroit lost a quarter-million residents between 2000 and 2010. A population that in the 1950s reached 1.8 million is struggling to stay above 700,000. Much of the middle-class and scores of businesses also have fled Detroit, taking their tax dollars with them.

Orr was unable to convince a host of creditors, the city's union and pension boards to take pennies on the dollar to help facilitate the city's massive financial restructuring.

He laid out his plans in June meetings with debt holders, in which his team warned there was a 50-50 chance of a bankruptcy filing.

Some creditors were asked to take about 10 cents on the dollar of what the city owed them. Underfunded pension claims would have received less than the 10 cents on the dollar under that plan.

A team of financial experts put together by Orr said that proposal was Detroit's one shot to permanently fix its fiscal problems.

If the bankruptcy filing is approved, city assets could be liquidated to satisfy demands for payment.

Detroit's budget deficit is believed to be more than $380 million. Orr has said long-term debt was more than $14 billion and could be between $17 billion and $20 billion.